The field of the invention generally relates to azimuth scanning radar systems, and more particularly relates to radar apparatus and method for enhancing the display of moving targets using scan-to-scan correlation.
As is well known, airport ground controllers need to keep track of aircraft and ground vehicles in and around an airport. For example, in order to insure that runways and taxiways are clear for traffic, controllers need to know the location of aircraft, and also whether they are taking off, landing, taxiing, or parking. During clear weather, ground controllers in the tower can rely heavily on visual observation of the aircraft and ground vehicles. However, during fog or other weather conditions of adverse visibility, parts or all of the airfield may not be clearly visible from the tower. Accordingly, during conditions of limited or adverse visibility, the ground controllers must rely heavily on the airport ground surveillance radar. Further, these are critical times because the visibility of pilots may often be obscured so there is a greater probability of a pilot moving his aircraft to an unintended or unauthorized location.
It is also well known that a pulse radar for such an airport installation typically has an antenna that scans in azimuth while transmitting pulses. When a transmitted pulse strikes an object, an echo or return is received back at the antenna and the time of receipt is a function of the range to the object from which it was reflected. Thus, the input to the radar receiver for one transmitted pulse can be categorized as a train of echoes or pulses from objects at different distances. Unfortunately, transmitted pulses reflect back from objects that are not of interest to the radar operator. For example, at an airport installation, echoes or returns are typically received from terminal buildings, runway lights, and ground clutter which may, for example, be returns from grassy areas surrounding the runways and taxiways. In fact, at a typical installation, the returns from objects that are not of interest vastly out number the returns from targets of interest. In other words, the display of the targets of interest may often be masked or cluttered by the display of returns from objects that are not of interest. This situation may make the radar display confusing to the operator thereby impeding the efficient performance of his job. The operator's task is further complicated by the fact that he should be primarily interested in targets that are actually moving (e.g. aircraft taxiing, landing, and taking off, as well as ground vehicles driving on the taxiways and apron areas) because these represent the greatest potential for collision.
U.S. Pat. No. 4,833,475 describes a pulse radar system that is particularly adapted for on-board operation in a marine environment. The system performs signal processing on echo return signals and, in response thereto, provides a raster scan display that is refreshed from an x,y bit image memory that has two bits corresponding to each picture element or pixel of the radar image so that each pixel can be displayed at different levels of brightness. The two bits for each pixel in the bit image memory are a function of the history of echo returns from the spacial location with which the pixel corresponds. More specifically, following some analog processing to reduce the effects of clutter returns, the echo signals are digitized into a plurality of successive single bit range cells or bins for each transmitted pulse. Then, a scan converter is used to provide a corresponding x,y address for each range cell so that the digitized data or video can be mapped to the respective x,y coordinates of the bit image memory. In particular, as the data for each range cell is read out of a range cell memory, the contents (digital level 00-11) of the corresponding bit image memory location is read and then modified in accordance with the new range cell video data. Next, the modified data is written back into the bit image memory. If the new video data indicates that an echo was detected, the level in the corresponding location of the bit image memory is incremented to a brighter level up to binary 11, and if no return was detected, the level is decremented. In such manner, scan-to-scan correlation is used to provide spacial discrimination of clutter. That is, due to the dynamic nature of clutter, and in particular sea clutter, there is only a small probability of receiving a return from the same spacial location on successive scans. Thus, because clutter does not tend to correlate positively from scan-to-scan, the clutter generally does not display at a bright, or enhanced level. However, targets that do have positive correlation from scan-to-scan are able to build up the stored level within the bit image memory, and thus their display is enhanced. This described technique, however, does not provide advantageous operation in the earlier described airport environment because moving targets which are of the most interest would not have positive scan-to-scan correlation. Thus, such moving targets would be displayed at reduced brightness.